Optical correlators fall into two main categories, spatial correlators, which are well known, and temporal correlators. The present device is a temporal correlator.
A typical prior art correlator 100, also known as a matched filter, adaptive filter, or transversal filter, is depicted in FIG. 1. Correlator 100 includes three elements: a tapped delay line 110, a series of weights sk, generally referred to by reference number 120 and a summer 130. Each tap produces a replica of an input signal with a delay that is some integer multiple of a basic delay increment τ. The weighting elements 110 are a series of phase shifters and/or amplitude changing elements. The summing device 130 is labeled Σ.
Each of the time-shifted replicas from the tapped delay line is multiplied by a weight, which may be either a phase weight, also known as a complex weight, or an amplitude weight, or a combination. In optical correlation, a processor is said to be coherent if the weights are complex and interference is used to combine the signals. A processor is said to be incoherent if the weights are amplitude-only.
The time-shifted and weighted signals are summed, and this combination of processes produces a correlation. Namely, the input signal is correlated with an arbitrary function that is implemented in the series of weights chosen. The resulting signal is a measure of how similar the incoming signal is to the reference signal encoded in the weights.
Optical correlators have been a topic of interest recently, especially for Optical Code Division Multiple Access (“OCMDA”). These correlators generally consist of a tapped delay line based on optical fiber. Two common prior art optical correlators are illustrated in FIG. 2. A first prior art optical correlator 210 employs a 1×N fiber splitter to generate N copies of an input signal, then each copy of the signal is delayed by a different length of fiber. A second prior art optical correlator 220 employs a series of 2×2 fiber optic splitters combined in various types of lattices. The lattice depicted in FIG. 2 is configured such that at each tap part of the signal takes the short path and part takes the long path, receiving a delay.
In these designs, however, a fiber splitter is required for each tap. Thus it becomes difficult to implement a large number of them. Such an optical correlator has a practical upper limit of about 100 taps, making the maximum practical resolution of such a correlator about 100 samples.
Prior art optical correlators have several disadvantages. One such disadvantage is that each fiber splitter results in some insertion loss. Another disadvantage of prior art optical correlators is that it is difficult to control the splitting ratio of each fiber optic splitter. Further, some type of tuning may be required to keep the amplitudes constant.
Consequently, a need exists for an optical correlator which addresses the disadvantages of the prior art and provides a greater number of taps.